One of the features of GC that, for some reason, I find highly annoying is the inclusion of an IVR for the receiving end (i.e., "Press 1 to accept the call, Press 2 to send it to voicemail, etc.). This gets in the way if you want to send inbound calls to your own IVR or a conference room. That's not to say that you can't work around it though....
You'll need to trick GC into thinking that you (as the receiving caller) punched "1" to accept the call. With Asterisk, it's easy:
1) Edit your previous configuration so that /etc/asterisk/extensions.conf has the following in the gizmo context:
exten => s,1,Answer exten => s,2,Wait(3) exten => s,3,SendDTMF(1) exten => s,4,Goto(default,201,1)
instead of
exten => s,1,Dial(SIP/1703)
Of course, "default,201" is the location in your dialplan where you want the call to automatically go. For me, 201 is the number for one of my conference rooms. It could just as easily be directed somewhere else. If you still wanted it to go to the hard phone but you want your Asterisk voicemail to pick up if the call doesn't go through, it'd look something like:
exten => s,1,Answer exten => s,2,Wait(3) exten => s,3,SendDTMF(1) exten => s,4,Dial(SIP/1703,20) exten => s,5,VoiceMail(1234,u)